The present invention is directed, in general, to surface acoustic wave (SAW) devices and, more specifically, to a system and method for dissipating static charge generated in a SAW device.
Electronic signal processing by way of SAW devices has been widely adopted by the electronics industry. SAW devices can be designed to operate as analog electrical filters that function at over a wide range of frequencies. They also have several advantages over conventional technologies.
SAW devices can be designed to provide complex signal processing in a single unit. Saw devices also benefit being able to be mass-produced using conventional semiconductor fabrication techniques, leading to devices with little variation from device to device at a low cost. Such devices can be integrated into many digital communications systems and can be designed to operate in high harmonic modes in the gigahertz (GHz) frequency range.
The response characteristics of a particular SAW device are governed by several factors. One is the geometry of conductors laid out on the SAW resonator""s piezoelectric substrate. A typical geometry for a SAW resonator includes first and second SAW finger sets. Portions of the finger sets are interdigitated in a central region of the SAW resonator and are employed to generate or attenuate acoustic waves. Additional non-interdigitated finger sets lie outside of the central region and serve to reflect acoustic waves back into the central region. Proper operation and containment of the acoustic waves require precise construction of both the central and outlying regions.
The interdigitated finger sets act as input and output signal ports when an AC voltage is applied to the signal input portion of the metal lines. Application of an appropriate input electrical AC signal provides the stimulus to create an acoustic wave that may typically be a Rayleigh wave with motion confined to about one acoustic wavelength under the free surface of the piezoelectric substrate. Alternatively, the acoustic excitation may be a xe2x80x9cleaky wave,xe2x80x9d which also finds application in modern radio frequency devices. This wave is propagates to the receiver portion. The fingers corresponding to the signal receiving portion draw energy from the acoustic wave in the lattice and convert it into a filtered electrical signal.
However, effective operation at high frequencies and general reduction in device size require a SAW resonator with smaller, more closely spaced finger sets. An undesirable effect of these small geometries in that the metal lines become subject to failure. One particularly troublesome mechanism of failure results from formation of charge in the piezoelectric substrate. During heating cycles during the manufacturing process, the piezoelectric substrate develops mobile charge carriers. These charge carriers then tend to migrate to the conductive finger sets and accumulate at areas of low electrical potential, such as defect sites. If the charge carriers accumulate sufficiently, they arc, damaging or destroying the ability of the interdigitated metal lines to transmit and detect the surface acoustic wave.
Accordingly, what is needed in the art is a surface acoustic wave device and a method of manufacturing a surface acoustic wave device that reduces or eliminates the damage to the device resulting from charge carriers in the substrate.
To address the above-discussed deficiencies of the prior art, the present invention provides a SAW device, a method of manufacturing the same and a SAW filter having at least one SAW device. In one embodiment, the SAW device includes: (1) a piezoelectric substrate, (2) a conductive layer located over the piezoelectric substrate and (3) a resistive layer, interposing a portion of the conductive layer and the piezoelectric substrate, that forms a return path for static charge migrating from the piezoelectric substrate to the conductive layer.
The present invention is based in part on the recognition that piezoelectric and pyroelectric effects inherent in the piezoelectric substrate manifest themselves during the manufacture of a SAW device by generating charge carriers. These charge carriers migrate from the substrate to the overlying conductive layer and collect at points of low electrical potential. If the density of these charge carriers reaches a threshold, an electrical arc may be formed that could harm or destroy part of the conductive layer or the substrate. This may render the device inoperative.
In response to this problem, the present invention provides a convenient path for returning static charge from the conductive layer back to the underlying substrate. In one embodiment, the conductive path takes the form of a resistive layer interposing the conductive layer and the substrate. The resistance presented by the resistive layer should (but need not) be sufficiently small to prevent a harmful collection of charge carriers in the conductive layer, but sufficiently large so as not materially to impair subsequent operation of the SAW device.
In one embodiment of the present invention, the piezoelectric substrate includes one selected from the group consisting of: (1) bismuth germanium oxide, (2) gallium arsenide, (3) lithium borate, (4) lithium niobate, (5) lithium tantalate, (6) langasite, (7) lead zirconium tantalate, and (8) quartz. Those skilled in the pertinent art will understand that other currently-known and later-discovered materials may be suitable for use as a substrate, depending upon a particular application.
In one embodiment of the present invention, the conductive layer includes one selected from the group consisting of: (1) aluminum, (2) copper, (3) gold, (4) silver, (5) platinum and (6) palladium. Those skilled in the pertinent art will understand that other materials may be suitable for use as a conductive later, depending upon a particular application.
In one embodiment of the present invention, the resistive layer includes one selected from the group consisting of: (1) doped silicon, (2) titanium, (3) zirconium, (4) hafnium, (5) vanadium, (6) niobium, (7) tantalum, (8) molybdenum, (9) tungsten, (10) chromium, (11) nitrides thereof and (12) carbides thereof. Those skilled in the pertinent art will understand that other materials may be suitable for use as a resistive layer, depending upon a particular application.
In one embodiment of the present invention, the resistive layer couples a selected signal pad to one of a plurality of ground pads. In an embodiment to be illustrated and described, the SAW device includes two signal pads and four ground pads and the resistive layer is divided into portions that span the two signal pads and the four ground pads. In the illustrated and described embodiment, four resistors are formed.
In one embodiment of the present invention, the resistive layer interposes an entirety of a pad portion of the conductive layer and the piezoelectric substrate. The resistive layer may alternatively interpose only a portion of the pad portion of the conductive layer or may actually underlie a finger portion. The resistive layer should advantageously not materially alter operation of the SAW device, however.
The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.